The present invention relates to the design and development of a new kind of polymeric material having a layered structure, a relatively high density, and good thermal and electrical insulation properties.
Polymer foams are widely used for thermal and electrical insulation; construction insulation boards and coaxial communication cables are typical examples of such uses. The blowing agent (defined as a compressed gas or vapor or liquid), used for making foam, or gas trapped in the foam-cells has a much lower thermal conductivity and a lower dielectric constant than the polymer. As a result, the thermal conductivity and dielectric constant of foams are significantly smaller than those of the parent material, and the lower the foam density, the greater the reduction in the thermal conductivity and dielectric constant. See Rodriguez-Perez et al.1 and Knott2. In addition to foam density, the cell geometry, including size and shape, and cell density, also affect the insulation properties. Thermal conductivity, for example, shows a significant dependence on cell geometry. See Harding.3 
Recently, polymer foams with very small cells have been developed for use in advanced and complex electronic devices. Nanofoams, for example, are being developed to manufacture chips with very high on-chip device densities. The target for the nanofoams is to achieve a dielectric constant of 2.0 (see, for example, Hedrick et al.4). In order to produce nanofoams, selected thermally labile blocks are introduced into a high glass transition temperature Tg polymer, such as polyimide. On heating, labile blocks undergo sharp and clean thermal decomposition at a temperature much below the Tg of the parent polymer to give a nanometer-sized closed-cell structure with about 20% void space.
Although foaming is the most popular way to use polymers for insulation purposes, the efficiency in achieving the desired insulation using a cellular morphology is, in fact, quite low. The reason is that the cell walls in polymer foams will always act as paths for energy transfer no matter how low the foam density. Furthermore, any improved performance in insulation comes at the expense of making cell walls thinner, resulting in low-density foams. However, low-density foams are mechanically and electrically weak, and dimensionally unstable.
It is an object of the present invention to develop polymeric materials that have high density and good thermal, electrical and sound insulation properties.
The present invention deals with such a new class of polymeric materials. These materials have a layered structure with an adjustable layer density, typically around 10 to 2000 layers/mm. The layers are from 0.05 to 100 xcexcm in thickness. The polymer layers are separated by discontinuous narrow gas-containing gaps. The size of the gaps can be controlled either to a few nanometers (to give nanolayered polymers) or to a few micrometers (to give microlayered polymers), depending on the process selected. Gap density, i.e. number of gaps per unit thickness, can be also used to describe the layered materials. Its value is almost the same as the layer density i.e. 10 to 2000 gaps/mm, and the gap width can be either xe2x89xa6100 nm (for nanolayered polymers) or xe2x89xa71 xcexcm (for microlayered polymers).
Another interesting property of the nanolayered polymers is that they lose the layered morphology and change back to regular structure at a certain temperature above the polymer""s Tg. For example, nanolayered PS has silvery appearance but if kept at 120xc2x0 C. for half an hour it becomes transparent, and the layer structure and interlayer gap in the material disappear. Accordingly, such materials could also be used as a temperature-sensitive smart-fuse or sensor. When the working temperature surpasses a certain preset security value, the materials lose their insulation properties.
The multilayered polymers according to the invention can be produced using two distinct process mechanisms, one for nanolayered polymers and another one for microlayered polymers. The nanolayer process involves a new concept of using a blowing agent to slice the polymer into layers. In this two-step process, the polymer is first built up from polymer pieces with a low degree of interfacial entanglement, that is, with a low degree of entanglements between polymer chains in adjacent pieces. This is followed by dissolving a selected blowing agent in the polymer matrix, and then subjecting the polymer-blowing agent solution to ambient pressure and a certain temperature which depends upon the polymer/blowing agent combination. The escaping blowing agent breaks apart the interfacial entanglements, resulting in a layered morphology. The properties and/or characteristics of the layered structure, including the layer thickness, the interconnections between the layers, and the interlayer gap, can be controlled in the process.
Two ways of introducing such inhomogeneous chain interfacial entanglements in polymers are: compression molding of stacked polymer films and compression molding of stacked polymer particles. It will be appreciated that other means could be employed for this purpose, such as ultrasonic welding, coextrusion, or other hot compression means.
Chain entanglements within a polymer film or polymer particles produced from the melt or solution state are usually high, homogeneous, and are not affected by the compression molding process. However, the polymer chain entanglement in the interfacial regions between the films or particles obtained by compression molding are not homogeneous and are significantly lower. The degree of polymer chain entanglement in the interfacial region depends on the interchain diffusion which, in turn, depends on the temperature, processing time, and pressure conditions used in the molding process. Thus, by appropriate control of these parameters, the molding process can produce materials with interfacial regions having a degree of polymer chain entanglement lower than that in the parent polymer material. The polymer with the low degree of interfacial polymer chain entanglement thus obtained is then exposed to a selected blowing agent to dissolve a certain amount of it. The selection of blowing agent depends upon the polymer, with various specific polymer/blowing agent combinations being preferred. When removed from the blowing agent and transferred into an ambient-pressure environment at a desired temperature, nanolayered morphology starts to develop in the material. It is important that the processing temperature at ambient pressure be below the Tg of the polymer-blowing agent system. Otherwise, microcellular foam structure will form in the whole material because the highly entangled parts also deform at a temperature above the Tg, allowing cells to nucleate and grow.
Microlayered polymers are produced in a different way. The process involves the use of stress-induced nucleation mechanism, which is the subject of our co-pending U.S. application Ser. No. 09/161,448, filed Sep. 28, 1998, to uniformly nucleate cells in the polymers containing dissolved blowing agent, and force the cells to grow in a certain direction. Briefly, a polymer can be exposed to a blowing agent until saturation, followed by depressurization and compression stressing at a temperature above the Tg of the polymer-blowing agent system. Cell nucleation starts instantly, followed by cell growth. When the applied stress is sufficiently high, cells tend to grow in a direction normal to the stress direction and some cells tend to be interconnected, resulting in a layered morphology with micrometer sized discontinuous gaps between the layers. In the extreme case when the applied stress is very high, the interlayer gaps become continuous and the material splits into several, completely separated thin layers.
According to one aspect of the invention an integral multi-layered polymer material is provided, comprising multiple layers of a polymeric material, and discontinuous gas-containing gaps between adjacent layers.
As will be appreciated hereinafter, the polymer material may be either a single polymer or a blend of compatible polymers.
According to another aspect of the invention, a process is provided for producing an integral multi-layered polymer, comprising
(a) welding together a plurality of pieces of a polymeric material at a selected pressure and temperature, for a time sufficient to introduce interfacial entanglements between polymer chains in adjacent pieces, such that the degree of interfacial chain entanglement is lower than that within the parent polymer,
(b) exposing the polymer material thus processed to a blowing agent to achieve a certain level of solubility of the blowing agent in the polymer, and
(c) removing the polymer from the blowing agent to an environment at a pressure of 0 to 2 atm and processing the polymer at a selected temperature below the Tg of the polymer/blowing agent combination, for a time sufficient to produce a multi-layered polymer of nano-layered morphology.
According to yet another aspect of the invention, a process is provided for producing an integral multi-layered polymer, comprising
(a) selecting a suitable polymer and blowing agent combination, wherein the polymer is in a solid or melt state, and the blowing agent is in the form of a gas or a volatile liquid,
(b) exposing the polymer to the blowing agent at a conditioning temperature, pressure and exposure time selected according to the thermodynamic properties of the polymer/blowing agent combination to form a polymer/blowing agent solution having a desired solubility up to the maximum saturation solubility of the blowing agent in the polymer,
(c) slowly depressurizing to ambient pressure to prevent cell nucleation,
(d) applying an external stress to the polymer/blowing agent solution at a temperature higher than the Tg of the polymer/blowing agent system, wherein the amount of stress applied is selected to give a multi-layered polymer material of microlayered morphology with or without closed or open cells in the polymer layers, and
(e) quenching the resulting polymer material by rapid cooling to a lower temperature.